Add repo restack

[gr4cza]

After something is merged to main, I tend to check out the trunk and run stack restack to update all stacks at once and keep them fresh.

It would be nice if we could restack everything without needing to check out the trunk itself, allowing us to continue working from where we left off after the restack.

[abhinav]

Possible solution:
The --restack flag for repo sync (#479) could accept an optional value, e.g. --restack=all to restack all tracked branches.
This could also include a configuration override (maybe spice.repoSync.restack) that would allow making that the default.

WDYT?

[gr4cza]

I wouldn't tie it completely to the sync it self. It would be more aligned with the other scopes (branch, stack, etc...) if we add a new one to repo.

It makes sense extend the --restack flag to be more versetile.

[abhinav]

Fair point. repo as a new scope for restack also makes sense.